Circuit for controlling separation and high-cut operation of a stereo demodulator in an FM radio receiver

ABSTRACT

A control circuit in an FM radio receiver includes a current mirror circuit which extracts a ripple component caused by a multipath disturbance and superposed on an S-meter output voltage from an intermediate frequency amplifier/detector of the FM radio receiver, and decreases the S-meter output voltage in response to the degree of the ripple component to use the decreased S-meter output voltage as a control voltage for control of separation and high-cut operations of a stereo demodulating stage of the FM radio receiver.

FIELD OF THE INVENTION

This invention relates to an FM radio receiver, and more particularly toan improvement of a control circuit which generates a d.c. voltage forcontrolling separation and high-cut operation of a stereo demodulator inan FM radio receiver.

BACKGROUND OF THE INVENTION

An FM radio receiver, in general, comprises a front end including atuner, intermediate frequency amplifier/detector, stereo demodulatingstage and audio stage. Japanese Patent Publication No. 15499/82 proposesa circuit which is used in such an FM radio receiver to automaticallycontrol the stereo demodulating stage and reduce a noise enteredtherein.

FIG. 4 is a block diagram of an FM radio receiver of this type in whichreference symbol ANT designates an antenna, 1 refers to a front end, 2to an intermediate frequency (IF) amplifier/detector, 3 to a stereodemodulating stage, 4 to an audio stage, 5 to right (R) and left (L)speakers, and 6 to a control circuit.

Assume now that an FM wave having a frequency f₀ is entered in thereceiver of FIG. 4 through the antenna ANT. The entered wave isconverted to an intermediate frequency signal having a frequencycomponent of 10.7 MHz by the front end 1. The intermediate frequencycomponent is amplified and FM-detected, and converted to an audiooutput. If it is a stereophonic broadcasting, the audio output isdivided into right (R) and left (L) components by the stereodemodulating stage 3. The audio output is amplified by the audio stage 4and transmitted to the speaker 5. The IF amplifier/detector 2 alsoincludes a means for generating a d.c. voltage responsive to the fieldintensity of the FM wave entered in the antenna ANT. The voltage isnormally called "signal meter voltage" (hereinafter called "S-metervoltage") because it drives a signal meter provided in the front panelof an FM radio receiver to indicate the field intensity. The stereodemodulating stage 3 effects among other things separation control andhigh-cut control to reduce multipath noises. The separation control andhigh-cut control operations are controlled by the S-meter voltage whichis closely related to the field intensity.

Assume here that a multipath disturbance occurs in which the FMtransmitter signals arrive at the antenna over two or more paths, onedirectly arriving with a frequency f₀ and the others involvingreflections from buildings or other obstacles having the same frequencyand arriving later. The S-meter voltage V_(S) momentarily decreases asshown in FIG. 5, and a ripple component V_(P) corresponding to thereflections are generated below the reference voltage V₀.

The separation control and high-cut control operations of the stereodemodulating stage 3 serve to improve the signal-to-noise (S/N) ratioupon a rapid degradation thereof due to a decreased field intensity ofthe FM stereo transmitter signals. More specifically, since the S/Nratio is improved by 22 dB during monophonic reception as compared tostereophonic reception, a great degradation in the S/N ratio can beprevented by separation control and high-cut control responsive to theelectric field intensity of the FM stereo transmitter signals. Thesecontrols are effected by the control circuit 6. However, the ripplecomponent V_(P) produced in the negative range during multipathreception as shown in FIG. 5 simply continues for a very limited time,and the voltage V_(S) immediately returns to the original value V₀.Therefore, it is difficult to effectively activate the separationcontrol and high-cut control in response to the ripple component V_(P).In this connection, it is necessary to provide a delay in the change ofthe voltage V_(S) following an increase or decrease of the ripplecomponent V_(P). More specifically, if a relatively long time isprovided between a voltage drop of V_(S) responsive to a multipathreception and restoration of the original voltage V₀, the separationcontrol and high-cut control can sufficiently follow the change of theripple component V_(P). FIG. 4 shows among others the control circuit 6,which decreases the S-meter control voltage in accordance with thedegree of the multipath disturbance.

One arrangement of the control circuit 6 is shown in FIG. 6 in whichreference numeral 7 denotes an S-meter voltage input terminal, 8 refersto an amplifier, 9 to a negative rectifier, 10 to an adder, 11 to aseparation/high-cut control voltage output terminal, and VR to avariable resistor. The ripple component V_(P) generated by a multipathdisturbance and superposed on the S-meter voltage V_(S) is extracted viaa capacitor C_(i) and amplified by the amplifier 8. The ripple componentV_(P) from the amplifier 8 is rectified by the rectifier 9 into anegative voltage which in turn is added to the S-meter voltage by theadder 10 to decrease the level of the original S-meter voltage into acontrol voltage V_(C) at the terminal 11. The control voltage V_(C)activates separation and high-cut control operations of the stereodemodulating stage to reduce noises caused by the multipath disturbanceand improve the S/N ratio.

FIG. 7 is a circuit diagram of the negative rectifier 9 and adder 10included in the circuit of FIG. 6. Reference numeral 12 denotes anoutput terminal of the variable resistor VR (FIG. 6) and 13 designatesan output terminal of the amplifier 8. The ripple component caused bythe multipath disturbance and amplified by the amplifier 8 passesthrough a capacitor C₃ and negative-rectified by diodes D₁ and D₂ so asto negatively charge a capacitor C₁. When the difference between thenegative voltage of the capacitor C₁ and a voltage at a point B becomeslarger than a threshold voltage V_(D) of a diode D₃, the capacitor C₂discharges through a resistor R₁ and the diode D₃. As the result, acurrent flows through resistors R₃ and R₂ to charge the capacitor C₂,and decreases the voltage V_(C) at a point C as shown in FIG. 8. Beforethe multipath disturbance component V_(P) reaches a degree (a), thediode D₃ maintains the control voltage V_(C) at V_(D), and after thecomponent V_(P) exceeds (a), the control voltage decreases due toconduction of the diode D₃. When the multipath disturbance componentV_(P) is (b), the control voltage V_(C) is V₁. In this case, the controlvoltage V_(C) may take different values V₂, for example, in addition toV₁ at a fixed degree of the multipath disturbance component V_(P),depending on the gain of the amplifier 8. That is, a voltage changeratio γ₀, i.e.: ##EQU1## varies depending on the gain of the amplifieras represented by: ##EQU2## The gain of the amplifier 8 cannot bechanged so much due to a restriction of a d.c. amplification ratioh_(FE). Also, the use of a larger resistance in the adder 10 to increasethe voltage drop invites an influence to the time constant fixed for adelayed charging of the capacitor C₂. Therefore, free selection of thevoltage change ratio γ₀ cannot be expected by changes of the gain of theamplifier 8 nor the resistance R₁ in the adder 10 in the prior artcircuit.

OBJECT OF THE INVENTION

It is therefore an object of the invention to provide a control circuitof an FM radio receiver which permits user's free selection of a voltagechanging ratio, i.e. the change in a control voltage with respect to thedegree of a multipath disturbance, which control voltage is obtained bydecreasing an S-meter voltage by an amount of a ripple component causedby the multipath disturbance and superposed on the S-meter voltage andis used to control known separating and high-cut operations of a stereodemodulating stage of the FM radio receiver.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a circuit forcontrolling separation and high-cut operation in a stereo demodulatingstage of an FM radio receiver comprising:

voltage-current conversion means for converting into a current a ripplecomponent voltage involved in an S-meter output voltage from anintermediate frequency amplifier/detector of said FM radio receiver;

a mirror circuit receiving said current from the voltage-currentconversion means as a first reference current;

level fixing means for setting the level of said S-meter output voltage;and

output means decreasing an output voltage from said level fixing meansin response to an output current from said current mirror circuit andoutputting the resulting voltage via a time constant circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a d.c. voltage control circuit embodyingthe invention;

FIG. 2 is a current characteristic diagram of a current mirror circuit;

FIG. 3 is a diagram showing the change in a control voltage with,respect to a ratio between resistances;

FIG. 4 is a block diagram of an FM receiver including a d.c. voltagecontrol circuit;

FIG. 5 shows a waveform of an S-meter voltage during multipathdisturbance;

FIG. 6 is a block diagram of a prior art d.c. voltage control circuit;

FIG. 7 is a circuit diagram of a negative rectifier and an adderinvolved; and

FIG. 8 is a diagram showing the relationship between the control voltageand a multipath disturbance component.

DETAILED DESCRIPTION

The invention is hereinbelow described in detail, referring to apreferred embodiment illustrated in the drawings.

FIG. 1 shows an arrangement of a control circuit embodying the inventionfor generating a high-cut & separation control voltage V_(C). Thecircuit generally comprises level shift circuits 14-15, voltage-currentconverting circuit 16, a current comparison circuit, and a currentmirror circuit (two-transistor current source) 18. The circuit furtherincludes transistors Q₁ through Q₄, resistors R₄ through R₆, fixedcurrent sources 19-20 and capacitors C₄ and C₅. The capacitor C₅ is usedto provide a delay in the change of the S-meter voltage responsive tothe ripple component V_(P) as described before.

When no multipath disturbance occurs, the S-meter voltage V_(S) isdivided by a variable resistor V_(R) into V₀ and level-shifted by ΔV inthe level shift circuit 14. The base voltage of the transistor Q₁becomes V₀ +ΔV, and a voltage V₀ +ΔV -V_(BEQ1) appears at the emitter ofthe NPN transistor Q₁. Therefore, the base voltage of the PNP transistorQ₂ becomes V₀ +ΔV -V_(BEQ1), and the emitter potential of same becomesV₀ +ΔV -V_(BEQ2). Its output is shifted by -ΔV in the level shiftcircuit 15 into V₀ +ΔV-V_(BEQ1) +V_(BEQ2) -ΔV. Assuming that V_(BEQ1)=V_(BEQ2) (=base-emitter voltage of the transistors Q₁ and Q₂), thecontrol voltage V_(C) at the terminal 11 becomes V₀.

When a multipath disturbance occurs, the ripple component V_(P)superposed on the S-meter voltage V_(S) is extracted by the capacitor C₄and converted to a current I₁ by the voltage-current converter 16. Thecurrent I₁ is compared to a reference current I₀ in the currentcomparison circuit 17. The current comparison is effected to provide athreshold value as the diode D₃ in the prior art provides a threshold.The difference current I₁ -I₀ of the comparison circuit 17 is applied asa reference current to the current mirror circuit 18 includingtransistors Q₃ and Q₄ to generate a mirror current I_(n) represented by:##EQU3##

The current I_(n) flows through the resistor R₄ and causes a voltagedrop I_(n) R₄. This causes the same voltage drop of I_(n) R₄ in thecontrol voltage V_(C) of the terminal 11. This is represented by:

    V.sub.C =V.sub.0 +ΔV-V.sub.BEQ1 -I.sub.n R.sub.4 +V.sub.BEQ2 -ΔV=V.sub.0 -I.sub.n R.sub.4 (∵V.sub.BEQ1 =V.sub.BEQ2)

Therefore, the mirror current I_(n) is controlled by the ratio betweenresistors R₅ and R₆. This means that the control voltage V_(C) may becontrolled as desired by a change of the ratio of the resistors.

In this case, the time constant for a change in the S-meter voltageV_(S) is fixed by a fixed current I₃ and capacitor C₅ and is representedby: ##EQU4## where V is the output voltage of the transistor Q₂ Sincethe time constant is not affected by the multipath conversion current,any change in the ratio between the resistors R₅ and R₆ to change thevoltage drop does not changes the time constant.

FIG. 2 shows the change in the current I_(n) by a solid line and thechange in the current difference I₁ -I₀ by a broken line with respect tothe multipath disturbance component V_(P) of the abscissa and thecurrent I₀ of the ordinate. FIG. 3 shows the relationship between themultipath disturbance component V_(P) of the abscissa and the controlvoltage V_(C) of the ordinate, taking the ratio R₄ /R₅ as a parameter.The inclination becomes larger as the ratio of the resistors increases.

As described, the invention circuit enables a user to select any desireddegree of separation and high-cut operations of a stereo demodulatingstage of an FM radio receiver by selecting the reduction amount of themultipath disturbance at a desired degree suitable for different areaswhere the FM wave condition varies, and this control never affects thetime constant provided for a delayed change in the control voltage forseparation and high-cut operations of the system.

The embodiments of the invention in which an exclusive property orpriviledge is claimed are defined as follows:
 1. In a stereo FM receiverhaving an S-meter responsive to a field strength indicating voltagesupplied thereto and a stereo demodulator responsive to a controlvoltage supplied thereto for controlling adjusting the separation andhigh-cut properties of said demodulator, the improvementcomprising:control voltage generating means for generating said controlvoltage in response to said field strength indicating voltage;voltage-current conversion means for converting to a ripple current aripple component voltage present in said field strength indicatingvoltage; a current mirror circuit responsive to said ripple current fromthe voltage-current conversion means for providing a mirror currentcorresponding thereto and having a controllably chosen ratio withrespect thereto; and output means including a time-constant circuit andcontrollably responsive to said mirror current from said current mirrorcircuit for decreasing said control voltage in response to increase insaid mirror current and outputting the resulting voltage via saidtime-constant circuit.
 2. The circuit of claim 1 further comprising acurrent comparison circuit provided between said voltage-currentconversion means and current mirror circuit, said current comparisoncircuit including one input for reception of a reference current and theother input for reception of said current from said voltage-currentconversion means.
 3. The circuit of claim 2 wherein said current mirrorcircuit includes at least two transistors and two resistors connected toindividual emitters of said transistors, said mirror current beingdetermined by the output current from said current comparison circuitand the ratio between said two resistors.